3.1. Effect of pH on the Adsorption of Heavy Metals by Biomass
In
Figure 1, the results are depicted concerning the point of zero charge (pH
pzc). It was observed that the pH
pzc of
C. vulgaris,
Scenedesmus sp., and
S. platensis were approximately 6.96, 6.80, and 6.99, respectively. Previous studies have indicated that at pH levels below pH
pzc, the functional groups on the surface of biomass are tightly bound with the H
3O
+ ions in the aqueous solution [
22]. However, as the pH surpasses the pH
pzc, the outer layer of the biomass undergoes deprotonation, exposing the negatively charged functional groups [
40,
41,
42]. The exposed functional groups can then interact with heavy metals, facilitating the formation of bonds [
22].
Furthermore, as depicted in
Figure 2, the adsorption of heavy metals onto
C. vulgaris, Scenedesmus sp., and
S. platensis showed a dependence on pH. Increasing pH led to an increase in the adsorption capacity of the biomass for Cd, Co, and Cu. This effect can be attributed to higher concentrations of H
3O
+ ions in the aqueous solution at lower pH levels, which compete with heavy metal cations for available adsorption sites, consequently limiting the number of unoccupied adsorption sites on the biomass surfaces [
43]. As the pH transition from acidic to alkaline, the concentration of hydronium ions in the solution decreases, reducing competition for adsorption sites. Additionally, it was noted that at pH values above 9.5, 9.0, and 7.5, precipitation, rather than adsorption, became the predominant removal mechanism for Cd, Co, and Cu, respectively.
3.2. FTIR Spectra
The FTIR spectrum of the microalgae is illustrated in
Figure 3. The band near 3300 cm
−1 is attributed to the N-H stretching vibrations in amides related to proteins and the O-H stretching in water [
44]. The region between 2800 cm
−1 and 3000 cm
−1 is indicative of the stretching vibrations of aliphatic C-H groups found in lipids and proteins, as well as the asymmetric stretching of aldehydes [
45]. The band at 1650 cm
−1 corresponds to the C=O stretching vibrations in protein-associated amides [
44]. The peak at 1527 cm
−1 is due to the C-N and N-H stretching vibrations in proteins, whereas the peak at 1470 cm
−1 is associated with C=C stretching [
44]. After adsorption, slight changes were observed for all three strains of microalgae, between wavenumbers corresponding to the O-H groups, the amine and amide groups, carboxyl functional groups, and C-O vibrations in the hydroxyl groups.
The spectra provide insight into the functional groups involved in the biosorption process. For instance, the intensity and position of the O-H and carboxyl group peaks showed noticeable shifts, indicating their involvement in the biosorption process. Specifically, the peak corresponding to the O-H stretching vibration showed a slight shift in all three strains of biomass after adsorption, suggesting interactions between the metal ions and hydroxyl groups on the biomass surface. These spectral changes imply the formation of metal–oxygen and metal–nitrogen bonds, corroborating the involvement of hydroxyl, carboxyl, and amine groups in the metal binding process. The shifts in wavenumber and changes in peak intensity confirm that functional groups on the biomass surface play a crucial role in the adsorption of heavy metals.
3.3. Biosorption under Acidic Conditions
The adsorption data of Cd, Co, and Cu exhibited good fits (0.87 < R
2 < 0.99) across all isotherm models tested with
C. vulgaris,
Scenedesmus sp., and
S. platensis under acidic conditions.
Figure 4 and
Table 1 show the biosorption isotherms and sorption parameters of Cd, Co, and Cu onto C.
vulgaris under acidic conditions, respectively. The adsorption of metal ions onto biomass was analyzed using the Langmuir model [
42,
43], which assumes monolayer adsorption on homogeneous surfaces. The determination coefficient (R
2) values for Cd, Co, and Cu obtained from the Langmuir model exceeded those from the Freundlich model, indicating that the sorption of the cations onto
C. vulgaris under acidic conditions was predominantly homogenous. The Langmuir model estimated the maximum adsorption capacity (
qmL) for
C. vulgaris in the following order: Co (2172 mmol/kg) > Cu (1917 mmol/kg) > Cd (1175 mmol/kg). The
RL parameter of the Langmuir model indicates the favorability of adsorption. It encompasses various scenarios: unfavorability (
RL > 1), linearity (
RL = 1), and favorability (0 <
RL < 1) [
38]. In this study, the
RL values were computed at the maximum initial concentration, and all produced values were consistent with
RL < 1, indicating that the sorption of Cd, Co, and Cu onto
C. vulgaris under acidic conditions was favorable.
The Freundlich model has been also employed to describe the sorption of cations onto biomass [
34]. It incorporates the heterogeneity of the adsorbent and the exponential distribution of the active sites and their corresponding energies. The
KF values in the Freundlich model describe the adsorption affinity of the adsorbent for the sorbent. As depicted in
Table 1, the
KF values for
C. vulgaris under acidic conditions followed the order Cu (609.8 mmol
1-NL
N/kg) > Co (483.7 mmol
1-NL
N/kg) > Cd (439.8 mmol
1-NL
N/kg).
The D-R model is mostly utilized for characterizing the sorption of molecules and ions onto microporous surfaces [
38]. While some researchers have applied the D-R model to elucidate cation adsorption onto microalgae, most researchers have focused on the Langmuir and Freundlich isotherm models. However, studies that employed the D-R model to fit their isotherm data revealed that this model could indeed be used to describe the adsorption of metal ions onto biomass [
33]. One advantage that the D-R model offers over the Langmuir and Freundlich models is that it provides an estimate of the sorption energy (
E), which may be used to predict the mechanisms involved in adsorption. When
E is between 8 kJ/mol and 16 kJ/mol, the sorption mechanism is predominantly ion exchange; when
E is less than 8 kJ/mol, the sorption mechanism is predominantly physical in nature; moreover, when
E is greater than 16 kJ/mol, the sorption mechanism is described as chemical sorption. As presented in
Table 1, it was observed that the sorption mechanism involved in the adsorption of Cd, Co, and Cu onto
C. vulgaris under acidic conditions is predominantly physical in nature. Studies have demonstrated that physical sorption mainly occurs due to weak van der Waals forces [
38]. The
qmD parameter of the D-R model may also be used to describe the maximum adsorption capacity (
qmax) of the metal ions onto biomass. The
qmax values of
C. vulgaris for Cd, Co, and Cu are 648 mmol/kg, 1383 mmol/kg, and 1347 mmol/kg, respectively.
As observed in
Table 1, there was a discrepancy between the
qmD and
qmL values. This difference is attributed to the fact that the Langmuir model considers the adsorption of metal ions onto the monolayer surface of the biomass to compute the
qmL, while the D-R model computes the
qmD by considering the amount of heavy metal ions adsorbed into the micro-pores of the biomass [
46]. The determination coefficients (R
2) of the Langmuir and the D-R model for each metal were compared to determine which isotherm model provided a more accurate estimate of the actual maximum adsorption capacity (
qmax).
The relatively high R
2 values obtained using the D-R model in this study were explained by the fact that microalgae cell walls have a fibrillary and amorphous structure, resulting in a sandy-like structure [
33].
Figure 5 and
Table 2 show the biosorption isotherms and sorption parameters of Cd, Co, and Cu onto
Scenedesmus sp. under acidic conditions, respectively. The maximum adsorption capacity values (
qmL) obtained from the Langmuir model were in the order Co (2182 mmol/kg) > Cu (2070 mmol/kg) > Cd (1027 mmol/kg). Although both the D-R model and the Langmuir model yielded the maximum adsorption capacity of the adsorbed metal ions onto
Scenedesmus sp. in the same order from highest to lowest (Co
2+ > Cu
2+ > Cd
2+), the D-R model estimated the amount of metal ions adsorbed to be 593 mmol/kg, 1398 mmol/kg, and 1474 mmol/kg for Cd
2+, Co
2+, and Cu
2+, respectively. It was observed that the adsorption isotherms of Co
2+ and Cu
2+ were better fitted with the D-R model with R
2 values of 0.97 for Co and 0.95 for Cu, compared to 0.90 for Co and 0.92 for Cu with the Langmuir model. It was, therefore, assumed that the
qmD parameter provided a more accurate estimate of the actual maximum adsorption capacity of Co and Cu onto
Scenedesmus sp. under acidic conditions.
The higher the value of the Langmuir model parameter
bL, the more the adsorbent surface is coated with firmly attached sorbate molecules. In this study, the
bL parameters showed that, in acidic conditions, Cd ions were more tightly attached to
Scenedesmus sp. biomass than Cu and Co cations. As presented in
Table 2, the
KF parameters described the sorption affinity of the cations towards
Scenedesmus sp. to be in the order of Cu > Co > Cd, and this was the same order observed with C.
vulgaris under acidic conditions. The Freundlich model described favorable sorption of Cd, Co, and Cu onto
Scenedesmus sp., with N values in the range of 0.1 < 1/N < 1. This was consistent with the Langmuir isotherm model, which also described favorable sorption for all the examined cations onto
Scenedesmus sp. with
RL values corresponding to 0 <
RL < 1. The sorption energy of Cd, Co, and Cu, as obtained using the D-R model, was 6.178 kJ/mol, 3.609 kJ/mol, and 3.894 kJ/mol, respectively. These findings showed that the sorption of the studied cations onto
Scenedesmus sp. Under acidic circumstances is mostly physisorption.
Figure 6 and
Table 3 show the biosorption isotherms and sorption parameters of Cd, Co, and Cu onto S.
platensis under acidic conditions, respectively. According to the
qmL parameters, the maximum sorption capacity of the metal ions onto
S. platensis was 1034 mmol/kg, 1768 mmol/kg, and 3503 mmol/kg for Cd, Co, and Cu, respectively. On the other hand, the
qmD parameter estimated the maximum adsorption capacity to be 639 mmol/kg for Cd, 1359 mmol/kg for Co, and 1754 mmol/kg for Cu. With the exception of Cu, the adsorption isotherms were better fitted with the Langmuir model. The sorption energies of all the cations were less than 8 kJ/mol, indicating that physical sorption was the primary sorption process involved in cation sorption onto
S. platensis unlike
C.
vulgaris and
Scenedesmus sp., where the biomass’ affinity towards the cations as indicated by the
KF parameter was in the order Cu > Co > Cd; moreover, it was observed that
S. platensis’s sorption affinity for the cations under acidic conditions was in the order of Co > Cu > Cd.
3.5. Biosorption under Neutral Conditions
The adsorption of Cd, Co, and Cu fitted well (0.81 < R2 ≤ 0.99) to all the isotherm models with C. vulgaris, Scenedesmus sp., and S. platensis under neutral conditions.
Figure 8 and
Table 4 show the biosorption isotherms and sorption parameters of Cd, Co, and Cu onto C.
vulgaris under neutral conditions, respectively. The
qmax values as computed using the Langmuir model were 1012 mmol/kg for Cd, 2007 mmol/kg for Co, and 2318 mmol/kg for Cu. The D-R model, on the other hand, estimated
qmax values of 643 mmol/kg, 1424 mmol/kg, and 1558 mmol/kg for Cd, Co, and Cu respectively. With the exception of Co cations, the sorption isotherms were observed to have fitted better with the Langmuir model than the D-R model.
For all the cations, the sorption affinity denoted by the Freundlich model parameter
KF was higher in the experiments conducted under neutral conditions than in studies conducted under acidic settings. Likewise, as the environment changed from acidic to neutral, the binding capacity denoted by the Langmuir model parameter
bL increased. The rise in sorption affinity and binding capacity in neutral settings was attributed to the decrease in H
3O
+ concentration with an increase in pH. Moreover, in neutral settings, the functional groups on
C. vulgaris were deprotonated, thereby, facilitating the binding of the heavy metals onto biomass [
43].
Even with the shift from acidic to neutral settings, the energy of sorption denoted by the D-R model parameter E remained below 8 kJ/mol, indicating that the sorption mechanism of Cd, Co, and Cu onto C. vulgaris is largely physical in nature in neutral conditions, just as in acidic conditions.
Figure 9 and
Table 5 show the biosorption isotherms and sorption parameters of Cd, Co, and Cu onto
Scenedesmus sp. under neutral conditions, respectively. The
qmax values as computed using the Langmuir model were 1044 mmol/kg for Cd, 2070 mmol/kg for Co, and 2037 mmol/kg for Cu. The D-R model, on the other hand, estimated
qmax values of 606 mmol/kg, 1389 mmol/kg, and 1332 mmol/kg for Cd, Co, and Cu, respectively. With respect to the determination coefficient (R
2), the Langmuir model produced a much more accurate estimate of the
qmax for Cd and Cu, whereas the D-R model estimated the
qmax for Co better.
As with C. vulgaris, similar trends were observed in the sorption mechanism of the cations onto Scenedesmus sp., with a change from acidic to neutral environments. The binding capacity (bL) of the cations increased with a shift in the environment from 0.68 (L/mmol) to 0.81 (L/mmol), 0.29 (L/mmol) to 0.34 (L/mmol), and 0.42 (L/mmol) to 0.59 (L/mmol) for Cd2+, Co2+ and Cu2+, respectively. The sorption affinity (KF) of Scenedesmus sp. also increased with a shift in the environment. Furthermore, as with C. vulgaris, physical sorption remained the primary sorption mechanism of the cations onto Scenedesmus sp. despite switching from acidic to neutral settings.
Figure 10 and
Table 6 show the biosorption isotherms and sorption parameters of Cd, Co, and Cu onto
S. platensis under neutral conditions, respectively. The Langmuir model computed
qmax values of 887 mmol/kg for Cd, 1790 mmol/kg for Co, and 1824 mmol/kg for Cu. The D-R model, in contrast, estimated
qmax values of 625 mmol/kg, 1349 mmol/kg, and 1318 mmol/kg for Cd, Co, and Cu, respectively. Compared to the D-R model, the R
2 was higher with the Langmuir model for all the cations, implying that the
qmax indicated by the Langmuir model was a much more reliable estimate. The Langmuir model appeared to fit the Co and Cu isotherm data better than the Freundlich model, whereas the Cd isotherm data fitted better with the Freundlich model.
Just as with C. vulgaris and Scenedesmus sp., S. platensis’ sorption affinity (KF) for the cations increased in neutral settings. Additionally, the sorption mechanism remained unaltered as the D-R model generated sorption energy values (E), corresponding to physio-sorption.